free editing a rigged character's pose similar to editing bones in edit mode
You can pose your character, then apply the armature modifier, so your deformed mesh will become a base mesh. Then you can select the armature and choose "Apply pose as rest pose", then add a new armature modifier to the mesh and select your armature in its "object" field. In this way the deformed mesh and the posed armature will act as "base mesh" and "rest pose", without having to rebuild the weights.

If you need to unparent a bone from another you have to do it in edit mode.

If you add new geometry to the mesh you will have to give some weight to the new vertices, in weight paint mode or directly in the vertex group tab of the properties panel.